1. Field of the Invention
The invention relates to methods for triggering a semiconductor switch through the use of an inductive transformer having a primary side and a secondary side, wherein the semiconductor switch to be triggered is disposed on the secondary side of the transformer, a switch potential of a given polarity applied to the primary side of the transformer leads to the switching of the semiconductor switch. Before electrical energy is supplied to the primary side of the transformer, effecting a switching operation at the semiconductor switch, magnetic energy that may be stored in the transformer is dissipated from the transformer with the aid of a circuit configuration for demagnetization of the transformer. The invention also relates to circuit configurations for triggering an inductive transformer triggering a semiconductor switch, including a control input, a logic circuit with at least one controllable switch for switching a voltage to the primary winding of the transformer, and at least one free-running element for switching a voltage of opposite polarity to the primary winding of the transformer.
2. Description of the Related Art
Such methods and circuit configurations are known in principle. Depending on the structure of the inductive transformer, they are used not only for potential-free triggering of semiconductor switches, but they are also used to increase the triggering current or triggering voltage. Such methods and circuit configurations are used in combinational circuits, and in particular in resonance converter combinational circuits, among others.
In known generic methods of that type, the semiconductor switch is typically triggered by a pulse-width modulated signal, optionally having a variable frequency and variable duty cycle. Unless countermeasures are taken, the direct current component of such trigger signals causes the transformer to become magnetically saturated, rendering purposeful triggering of the semiconductor switch impossible.
Among other sources, an article entitled "Zuverlassiger Betrieb von MOSFETs in Bruckenschaltungen" [Reliable Operation of MOSFETs in Bridge Circuits] by H. R. Hassig and P. Zeller, in Elektronik [Electronics] No. 10, 1989, pp. 55-63, and in particular FIG. 1b on page 56, discloses a method for triggering a semiconductor switch in which a trigger signal is capacitively coupled in to the inductive transformer. The capacitor used in that method keeps the DC component away from the primary side of the transformer. The capacitor is charged to a DC potential corresponding to the mean value of the trigger signal, and it sends alternating current signals that are superimposed on the DC potential, to the transformer. The voltage amplitudes of the AC signals are accordingly dependent on the level of the DC potential. In such configurations, changing the duty cycle of the trigger signal changes the DC component at the capacitor, and thus the amplitude of the trigger voltage at the semiconductor switch changes as well.
The publication entitled "Technische Mitteilung aus dem Bereich Bauelemente, Integrierte Schaltnetzteil-Steuerschaltungen mit TDA 4700/TDA 4718, Funktion und Anwendung" [Technical Report on Components, Integrated Combinational Circuit Control Circuits with TDA 4700/TDA 4718, Function and Use] of the firm Siemens, Munich, 1980, particularly FIG. 19 on page 29, discloses another method of that generic type. In that method, an inductive transformer having an additional demagnetizing coil is used. The additional demagnetizing coil is connected, in particular, through a diode to a fixed potential. Upon a corresponding change of the switching state of the semiconductor caused by the triggering, the field energy introduced into the transformer upon pulse transmission is caused to decay through the demagnetizing coil. Since the demagnetizing coil forms a resonant circuit together with parasitic circuit capacitors such as the input capacitor of the semiconductor switch, the field energy is typically made to decay periodically. In order to prevent a resultant unintended switching of the semiconductor switch, the secondary circuit of the inductive transformer must be correspondingly damped. Such damping of the secondary circuit of the transformer causes power losses, and must also be dimensioned in accordance with both the inductance and the capacitance that are operative in demagnetizing operations.
It is accordingly an object of the invention to provide a method and a circuit configuration for triggering a semiconductor switch through the use of an inductive transformer, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, in which the amplitude of the trigger voltage is not dependent on the duty cycle of the trigger signal and in which unintentional switching of the semiconductor switch caused by periodic demagnetization of the transformer is prevented.